The present invention generally relates to a rigid ceiling supported within a closure structure and, more particularly, is concerned with a rigid ceiling suspended from and sealed within a closure structure for reducing the volume over a vessel.
A variety of gases are released from vessels containing pools of waste, petroleum and other stored liquids and semi-solids. Over time the content of these gases will increase within the internal volume of a closure structure above the surface of the pool. This volume of gases (such as H.sub.2 S or methane) must periodically be cleaned to reduce odors or so that regulated gas contents do not exceed certain prescribed standards. It is therefore desirable that the volume within the closure structure over the pool of waste be kept as small as possible to minimize the volume of gases which must be periodically cleansed and therefore save on cleaning costs.
One type of closure structure which can be used is internally supported. However, it is not desirable to build a closure structure which will require columns or other types of supports which run vertically up from the pool of waste to support the roof of the closure structure. First of all, the pool of waste may require agitation and the pool is easier to agitate when there is no interference from supporting structures. Secondly, supporting structures in contact with the pool create a location which encourages the buildup of waste within the pool. For these reasons it is desirable to build a self-supporting closure structure over the pool of waste.
Domes and vaults are closure structures which can be constructed to be self-supporting over large areas. However, the structure of vaults and domes creates a larger volume within the closure structure than is created by internally supported closure structures.
Prior practices have described ceilings installed within a dome or self-supporting structure. Apparatus has also been described for placing a floating deck within a vessel to prevent vapor losses.
U.S. Pat. No. 1,825,800 to Houseman discloses a frame capable of supporting the weight of a ceiling and a roof covering without the use of intermediate vertical supporting posts. A false ceiling may be suspended from the roof. The ceiling is designed to insulate the inside area of the enclosure from overhead noises.
U.S. Pat. No. 2,341,548 to Heineman described a diaphragm which extends across the top of a container below and spaced from a roof. An upper compartment is completely sealed such that the placement of a liquid in the upper compartment will decrease the temperature variations of the contents of the lower compartment due to absorption of the heat of vaporization during temperature rises and due to the return of this heat, by condensation, as the temperature drops.
U.S. Pat. No. 3,279,606 to Cox discloses a gas dome for anaerobic digesters including a rain shed and a ceiling plate. Trusses extend between the rain shed and ceiling plate. A gas deflector skirt or rim plate may be used to prevent the escape of any gas between the cover and the tank wall. The cover is dome shaped so that sludge gas generated during the digestion of the sludge is directed toward the central gas dome.
The publication entitled "Geodesic-Dome Tank Roof Cuts Water Contamination, Vapor Losses" from the Oil and Gas Journal describes the use of a floating deck within a tank. The floating deck is sealed around the rim to prevent vapor emissions.
None of the prior art patents are designed to fulfill the needs met by the present invention. These inventions are not designed to maintain an airtight seal within a closure structure over and above a deposit of waste. The patent to Houseman is relevant only in that it discloses a ceiling suspended from a roof. The patent to Heineman does not disclose the manner in which the diaphragm will specifically be constructed and how such diaphragm will achieve a seal. Heineman also fails to disclose a manner for maintaining a seal during conditions of distortion of the container. The patent to Cox does not disclose apparatus for suspending a ceiling from the closure structure, the structure of a lightweight ceiling which provides an effective seal and a manner of maintaining a seal around the edge of the ceiling while allowing for motion of the ceiling relative to adjacent structures.
The floating cover art has several disadvantages. Since floating covers are constructed to move independent of the vessel, they are not gas tight around the periphery of the cover. Floating covers also have a short useful service life, require periodic cleaning and act as catch basins for airborne debris and trash. Floating covers also obstruct equipment designed to interact with the substance contained in the vessel.